Oligodendrocytes play a vital physiological role in support of the central nervous system. Availability of oligodendrocytes for human therapy may facilitate healing of disabling conditions that result from defects in the myelin sheath that insulates nerve cells.
Multiple Sclerosis is a progressive and disabling demyelination disease, involving gradual destruction of the myelin sheath that surrounds the nerve cells in the brain and spinal cord. Symptoms range from numbness, visual impairment and cognitive changes, to paralysis. The disease is believed to have immunological and genetic components, often appearing in clinical form between ages 20 and 40. It affects about 300,000 people in the U.S. alone. Therapeutic modalities currently involve courses of β-interferon or corticosteroids. These drugs may shorten the symptomatic period during attacks, but generally do not prevent long-term disability.
Traumatic injury to the spinal cord causes demyelination of intact axons near the trauma site, which robs them of their capacity for neural transmission. There are about 11,000 new cases of spinal cord injury every year in the U.S. The SCI Information Network estimates that the lifetime direct costs to patients suffering from incomplete motor function at any level ranges from $400,000 to $2,200,000, not including lost wages and effects on the quality of life.
Myelin on cells of the central nervous system is put in place by oligodendrocytes, which wrap around axons in the vicinity, forming a myelin sheath. The role of oligodendrocytes and their progenitors in disease conditions is reviewed by Keirstead & Blakemore (Adv. Exp. Med. Biol. 468:183, 1999). Oligodendrocyte progenitors known as O-2A cells are present in normal adult CNS and in lesions of Multiple Sclerosis, and participate in remyelination (Scolding et al., Brain 121:2221, 1998; and Scolding et al., Neuroscience 89:1, 1999). Failure to remyelinate adequately may occur because oligodendrocytes proliferate symmetrically, using up the reservoir of progenitor cells where the damage is extensive.
Considerable research work has been done with a view to creating cell populations that could be used in regenerative medicine to restore neurological function (reviewed by Park et al., J. Neurotrauma 16:675, 1999). Keirstead et al. (J. Neuroscience 19:7529, 1999) isolated CNS precursors from the postnatal rat brain that generate oligodendrocytes and Schwann cells after transplantation. Svendsen et al. (J. Neurosci. Meth. 85:141, 1998) isolated precursor cells from the developing human cortex. Mujtaba et al. (Dev. Biol. 214:113, 1999) report isolation of neural precursors from embryonic stem cells.
PCT publication WO 97/07200 (Stanford U.) shows cultures of oligodendrocyte precursors isolated from adult rat brain. PCT publication WO 01/28342 (Washington U.) proposes certain methods for culturing nerve cells in preconditioned oligodendrocyte culture medium. U.S. Pat. No. 5,753,506 (Johe, CNS Stem Cell Technology) relates to a culture system for maintaining stem cells isolated from neural tissue that have the capacity to differentiate into neurons, astrocytes, and oligodendrocytes. U.S. Pat. No. 6,238,922 (StemCells Inc.) proposes dissociation of neural tissue into cells that have the capability of differentiating into neurons and glia. U.S. Pat. No. 6,235,527 (Rao et al., U. Utah) relates to populations of mammalian CNS glial-restricted precursor cells dissociated from mammalian neural tube tissue and selected based on the A2B5 cell surface marker.
U.S. Pat. No. 5,968,829 (Cytotherapeutics) claims culture medium containing CNS neural stem cells that have the capacity to produce neurons, astrocytes, and oligodendrocytes. PCT publication WO 97/32608 pertains to genetically engineered primary oligodendrocytes fro transplantation-mediated delivery in the CNS. U.S. Pat. No. 5,830,621 (Signal Pharmaceuticals) describes a human oligodendrocyte cell line deposited with the ATCC under Accession No. CRL 11881. The line is essentially free of the characteristic markers GFAP, GalC, O4, and A2B5.
Unfortunately, it is not yet clear whether progenitors isolated from neural tissue will have sufficient replicative capacity to produce the number of cells necessary for human clinical therapy.
An alternative source is pluripotent cells isolated from early embryonic tissue. Embryonic stem (ES) cells were first isolated from mouse embryos over 25 years ago (G. R. Martin, Proc. Natl. Acad. Sci. U.S.A. 78:7634, 1981). ES cells are believed to be capable of giving rise to progeny of virtually any tissue type of the same species.
Fraichard et al. (J. Cell Sci. 108:3181, 1995) report in vitro differentiation of mouse ES cells into glial cells and functional neurons. Mujtaba et al. (Dev. Biol. 214:113, 1999) report isolation of neural precursors from mouse ES cells. Li, Smith et al. (Cur. Biol. 8:971, 1998) report generation of neuronal precursors from mouse ES cells by lineage selection. Brustle, McKay et al. (Proc. Natl. Acad. Sci. USA 94:14809, 1997; Science 285:754, 1999) report glial precursors derived from mouse ES cells as a potential source of myelinating transplants. McDonald et al. (Nat. Med. 5:1410, 1999; Proc. Natl. Acad. Sci. USA 97:6126, 2000) report that mouse ES cells form oligodendrocytes and myelinate in culture and after spinal cord transplantation. Human ES cells were not isolated until much more recently (Thomson et al., Science 282:114, 1998). Human ES cells require very different conditions to keep them in an undifferentiated state, or direct them along particular differentiation pathways (U.S. Pat. Nos. 6,090,622 & 6,200,806; PCT publications WO 99/20741 & WO 01/51616). For this reason, much less is known about how to prepare relatively homogeneous populations of differentiated cells from human ES cells.
PCT publication WO 01/88104 (Carpenter, Geron Corporation) describes neural progenitor cell populations obtained by differentiating human ES cells. Populations have been obtained that are over 90% NCAM positive, 35% β-tubulin positive, and 75% A2B5 positive. Zhang et al. (Nature Biotech. 19:1129, 2001) report differentiation of transplantable neural precursors from human ES cells. International Patent Application PCT/US02/19477 (Carpenter et al., Geron Corporation) describes ES cell derived neural cell populations in which at least 10% of the MAP-2 positive cells in the produced population express tyrosine hydroxylase, a marker for dopaminergic neurons.
Recently, Billon et al. (J. Cell Sci. 115:3657, 2002) describe the timing of oligodendrocyte development from genetically engineered mouse ES cells. Kuo et al. (Biol. Reprod. Dec. 11, 2002) report a population of monkey ES derived cells that were 28% GFAP positive; and Xian et al. (Stem Cells 21:41, 2003) report generation of oligodendrocytes from mouse ES cells using the lineage-specific transcription factor Olig2.
In order to realize the full potential of pPS cells in the management of human health and disease, it is necessary to develop new paradigms to generate enriched populations of cells useful for treating demyelination conditions.